1. Field of the Invention
This invention relates to current mirrors and finds application, for example, in charge pump circuits.
2. Description of the Related Art
A conventional current mirror has an input or a reference device, which is usually diode-connected, and an output device. For increased accuracy the diode connection may be replaced by an amplifier which senses the voltages on the output electrodes of the reference and output devices and provides an output to the control electrodes of the devices.
Referring to FIG. 1, a simple current mirror comprises a reference device 1 (sometimes termed an input or logging device), and an output device 2. The devices shown are of the NMOS type, although it will be appreciated that other devices such as PMOS or bipolar devices may be employed. The drain of device 1 is connected to the gates of devices 1 and 2. The sources of devices 1 and 2 are connected to, in this case, ground. As will be appreciated, device 1 is diode-connected. The operation of such a current mirror is well known.
Referring now to FIG. 2, a current mirror having increased accuracy comprises a reference device 21, an output device 22, and an amplifier 23. Although devices 21 and 22 are shown as of the NMOS type, other devices may be used, such as PMOS or bipolar types. The sources of devices 21 and 22 are connected to, in this case, ground, and their gates are connected together. The drain of device 21 is connected to one input of amplifier 23. The drain of device 22 is connected to an output terminal OUT and to another input of amplifier 23. In operation, amplifier 23 senses the drain voltages of devices 21 and 22 and provides a control voltage to the gates of devices 21 and 22.